This invention relates to a magnet field type rotary electric apparatus such as an electric motor and a generator and more particularly to an arrangement for improving the performance of such apparatus.
It is known with this type of apparatus that is generally comprised of cooperating, relatively rotatable components one of which is provided with a plural number of circumferentially spaced permanent magnets and the other of which is provided with pole teeth wound with electrical coils reducing the cogging torque is desirable for smooth rotation with low noise and vibration. The cogging torque is produced in relation to the boundary areas and arrangement of the permanent magnets relative to the pole teeth and their coils. Various types of arrangements have been proposed for this purpose. However more can and should be done to improve performance.
For example, reference will be made to several prior art dc motor construction intended for reducing the cogging torque and which still leave much to be desired. For a first example of a prior art attempt, Japanese Published Patent Application JP-A-Hei11-136886 shows an arrangement where the permanent magnets are arranged to in a skewed relation to reduce cogging torque. This construction employs a plural number of approximately arcuate shaped permanent magnet segments spaced apart by gaps. The skew angle that intersects the gap between the permanent magnets is set smaller than the skew angles do not intersect so as to reduce cogging torque. With this type of solution, the achievement of magnetizing with varied skew angles and good accuracy is difficult.
Japanese Published Patent Application JP-A-2001-359266 shows another attempt at reducing the cogging torque by dividing the rotor is axially into a plural number of parts with each part displaced circumferentially around the rotational axis by a mechanical angle corresponding to a half period in relation to each cogging torque. A problem with this solution is that the rotor is merely divided into two parts in the axial direction and displaced by half the period of cogging torque. It cannot be divided into a greater number of stages and also determine an appropriate displacement angle. Therefore it cannot make an effective reduction in the cogging torque.
Yet another attempt is shown in Japanese Published Patent Application JP-A2000-308287 where the rotor core is divided into a plural number of blocks, with each block displaced in the direction of rotation and a thin plate of a non-magnetic material is interposed between the blocks to make an integral body. This construction prevents magnetic flux from leaking among the divided blocks of the rotor core. However, the displacement angle between the number of division stages cannot be determined accurately and effective reduction in the cogging torque cannot be made.
In addition to the aforenoted defects in the prior art constructions, suppression of distortion of the back electromotive force waveform is not considered. Thus performance is not significantly improved because of this added problem.
Therefore it is a principal object of this invention to provide an arrangement for not only reducing cogging torque but also suppressing the distortion of the back electromotive force waveform.